The present invention relates to the field of thermal imaging cameras and in particular to improvements of such cameras for detecting medium wave infrared and long wave infrared regions of the electromagnetic spectrum.
A principal application for thermal imaging cameras is the detection, recognition and subsequent identification (DRI) of objects. Present cameras are required to render to a display screen or to an “image processor” the “shape and texture” attributes of such objects and their contexts to such a quality that a human observer or an electronic substitute may perform these tasks to a high probability of success. The resolution of such devices is limited to the ability of humans, or their electronic substitute, to recognise objects from the rendering on a display screen.
When combined with the achievable performances of cameras and human observers and processors, these requirements frequently impose limits on the camera's maximum field of view to such an extent that the ranges at which the tasks of DRI can be achieved are incompatible with many applications of the camera. Within the limits of technology and those imposed by natural laws, an increase in the “task achievement range” requires a reduction in the field of view of the camera. With this narrowing field-of-view, the probability of an object being present in the field is reduced. Furthermore, any decrease in the field of view of the camera is likely to result in an increase in the area of the optical aperture with consequent impact on the cost and vulnerability of the optics and the aerodynamic performance of any aircraft on which the camera is deployed.
If the intended application requires a minimum field-of-view, then the ability of the camera to recognise objects is adversely affected and the camera has only sufficient resolving power to detect objects. Such a camera is then limited in its ability to discriminate between objects because the context will inevitably contain multiple features such as animals, heated rocks or vegetation that have the same temperature difference as that created by the genuine object. In such a situation, the application of the camera is limited by erroneous recognition.
The prior art teaches of thermal cameras characterised by a wide field-of-view and a low erroneous recognition rate. Such devices are employed for the measurements of the spectral emissivity of natural and cultural objects in the so-called Medium Waved Infrared (MWIR), between 3.2 um and 5.5 pm, and Long Wave Infrared (LWIR), 7.8, um and 11.4 um, atmospheric windows. It is known to those skilled in the art that the use of such a camera capable of measuring these attributes enhances the observer's ability to discriminate between classes of object such as trees, rocks, grasses and vehicles.
A thermal imaging camera with such a capability is known as a hyperspectral camera. Rather than observing the scene using a single waveband and presenting the image as a plane, the scene is decomposed into a number of planes representing spectral sub-bands or spectral bins. The assembly of these planes is then known as a “hyperspectral cube”.
It is well known to those skilled in the art and science that such hyperspectral cameras present difficulties in achieving adequate signal to noise ratio (SNR) against objects of interest whose temperature difference relative to the background is typically only a few Celsius. In a perfect thermal imaging camera, the noise in the instrument is dominated by that from the detector. To achieve such a performance, the noise internal to the detector itself must be made extremely low. This can only be achieved in detectors sensitive to LWIR radiation by cryogenically cooling the detector. Modern detectors are integrated with a closed-cycle cooling engine which can reduce the temperature of the detector array to values lower than 80 Kelvins. When fitted with such a detector, the camera is then capable of achieving “Background Limited”thermal sensitivity. This performance level indicates that the noise in the camera is created by the random arrival of photons from all objects in the field-of-view of the detector. The photon rate, and the fluctuation thereof, are determined by the temperature of the objects. As that temperature falls, so does the noise level in the detector.
This effect is exploited in modern, high performance, infrared detectors by engineering the detector package and cooling engine to cool not only the detector array but also a “cold-shield” enclosing the detector array.
The cold-shield is pierced to allow the detector to receive the scene image-forming rays from the imaging system such as a sequence of lenses or mirrors.
Inconsiderate design of this optical system leads to an instrument whose detector is exposed not only to radiation from the scene but also to that from the interior of the camera. Contributions to this additional radiation come either from the optical elements or from the enclosure, either directly or by reflections thereof from the optical components.
If the camera design is such that spectral filtering is provided prior to this process of intrusion by stray radiation, the SNR of the instrument will be adversely affected and will not achieve that possible if both the signal and noise had been spectrally filtered.
Prior designs of hyperspectral thermal cameras have solved this problem in a number of ways. A choice between the various methods is mainly influenced by the requirements of spectral resolving power and the operating waveband. The ratio of the operating waveband to the spectral resolving power is described by the term “number of channels” or “number of spectral bins”.
For a camera with only a modest number of spectral bins, a preferred method is to introduce a carousel of dielectric interference filters at the entrance window of the detector. Rotation of the carousel allows measurements of the radiation transmitted through the filter. The advantage of this method is that out-of band radiation is reflected from the filter out to the optical system and either absorbed in the camera body or reflected out of the camera. Thus, the noise from the camera optics is also filtered. Another advantage of this method is that a full spatial frame is gathered during the dwell time of the filter. The disadvantage of this method is that the behaviour of interference filters is very dependent upon the angle of arrival of rays.
Thus when used with focusing optics, the spectral bandpass of the filter is widened and the number of spectral bins is limited to less than about 8 in the LWIR band.
Higher spectral resolving power can be achieved by using a spectrally dispersive component such as a prism or a diffraction grating. The principal disadvantage of a prism instrument is that the dispersive power of prisms is relatively low so that long focal lengths and thus bulky imaging optics are required to form a usefully sized spectrum. In addition, light from the interior of the camera is uncontrolled and will increase the noise.
Thus, it is normal for such instruments optical components to be cooled to a very low temperature such that this intrusive radiation is reduced. In the very highest quality instruments it is normal to cool the entire instrument which may weigh 100 kg with a cryogenic liquid such as helium. This cooling requirement eliminates such instruments from large-scale deployment that requires maneuverability. The reflective diffraction grating has a very high dispersive power and is widely used in laboratory instruments, but these are also bulky. The oblique configuration of the instruments using reflection diffractive gratings also limits their use to optics with relatively poor light-gathering capacity and field-of-view at which high image quality is possible.
The highest spectral resolving power is achieved with an instrument using a variable optical path interferometer.
This capability is gained at the penalty of poor light gathering capacity and extreme sensitivity to relative mechanical motions of the camera components.